The $1.7 billion deal will pay Amegy holders cash and stock worth about $23.32 a share. The shares closed at $22.98 Tuesday amid rumors of a transaction; they traded below $17 as recently as June 15.

At $6.6 billion, Zions' market capitalization is roughly four times that of Amegy. The bank holding company runs about 400 branches in eight Western states. Amegy, meanwhile, operates about 75 branches around Houston and Dallas.